Disc drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disc medium, the actual information being stored in the form of magnetic transitions within the medium. The discs themselves are rotatably mounted on a spindle, the information being accessed by means of transducers located on a pivoting arm which moves radially over the surface of the disc. The read/write heads or transducers must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of information; thus the discs must be rotationally stable.
During operation, the discs are rotated at very high speeds within an enclosed housing by means of an electric motor which is generally located inside the hub or below the discs. One type of motor in common use is known as an in-hub or in-spindle motor. Such in-spindle motors typically have a spindle mounted by means of a bearing system to a motorshaft disposed in the center of the hub. In many well-established designs, two ball bearings are used. One of the bearings is typically located near the top of the spindle, and the other near the bottom. These bearings allow for rotational movement between the shaft and hub, while maintaining accurate alignment of the spindle to the shaft. The bearings themselves are normally lubricated by grease or oil.
The conventional bearing system described above, however, is prone to several shortcomings. First is the problem of vibration generated by the balls rolling on the raceways. Ball bearings used in hard disc drive spindles run under conditions that generally guarantee physical contact between raceway and ball, in spite of the lubrication layer provided by the bearing oil or grease. Hence, bearing balls running on the generally smooth but microscopically uneven and rough raceways, transmit this surface structure as well as their imperfection in sphericity in the form of vibration to the rotating disc. This vibration results in misalignment between the data tracks and the read/write transducer, limiting the data track density and the overall performance of the disc drive system.
Another problem is related to the application of hard disc drives in portable computer equipment and resulting requirements in shock resistance. Shocks create relative acceleration between the discs and the drive casting which in turn show up as a force across the bearing system. Since the contact surfaces in ball bearings are very small, the resulting contact pressures may exceed the yield strength of the bearing material, and cause long term deformation and damage to the raceway and the balls of the ball bearing.
Moreover, mechanical bearings are not easily scaleable to smaller dimensions. This is a significant drawback since the tendency in the disc drive industry has been to continually shrink the physical dimensions of the disc drive unit.
As an alternative to conventional ball bearing spindle systems, researchers have concentrated much of their efforts on developing a hydrodynamic bearing.
In these types of systems, lubricating fluid--either gas or liquid--functions as the actual bearing surface between the rotating spindle or rotating hub of the motor. For example, liquid lubricants comprising oil, more complex ferro-magnetic fluids or even air have been utilized in hydrodynamic bearing systems. The reason for the popularity of the use of air is the importance of avoiding the outgassing of contaminants into the sealed area of the head/disc housing. However, air does not provide the lubricating qualities of oil. The relatively high viscosity of oil allows for larger bearing gaps and therefore greater tolerances to achieve similar dynamic performance.
The lubricating fluid itself must be very accurately filled in the bearing. If the bearing is loaded with too much fluid, the fluid will inevitably escape into the surrounding atmosphere landing on the surface of the disc and inevitably degrade the performance of the disc drive. If too little fluid is loaded, then the physical surfaces of the spindle and housing will probably contact one another, leading to increased wear and eventual failure of the bearing system.
The current oil fill method for most hydrodynamic or fluid dynamic motors requires a complex and costly oil fill machine that requires a significant amount of skill and effort to maintain. It requires that a high level of vacuum be drawn on an assembled frame/sleeve with installed shaft. Because of the very tight clearances and sharp comers required for a fluid bearing, it has proven difficult to consistently evacuate all traces of air from the bearing before it is filled. The consequences of this are bubbles in the bearing that could lead to a shortened bearing life.
In addition to the complexities of reliably filling the motor (resulting in high process control costs) the current method leaves a considerable amount of excess oil on the surfaces of the sleeve which must subsequently be removed through an arduous post-cleaning process. This process frequently amounts to nearly one-third of the total assembly cost of the motor. Therefore, a more reliable assembly process which results in more accurate filling of the motor and less cleaning time is needed.